Double‐peaked Broad Emission Lines in NGC 4450 and Other LINERs

Spectra taken with HST reveal that NGC 4450 emits Balmer emission lines withdisplaced double peaks and extremely high-velocity wings. This characteristicline profile, previously seen in a few nearby LINERs and in a small fraction ofbroad-line radio galaxies, can be interpreted as a kinematic signature of arelativistic accretion disk. We can reproduce the observed profile with a modelfor a disk with a radial range of 1000-2000 gravitational radii and inclined by27 degrees along the line of sight. The small-aperture HST data also allow usto detect, for the first time, the featureless continuum at optical wavelengthsin NGC 4450; the nonstellar nucleus is intrinsically very faint, with M_B =-11.2 mag for D = 16.8 Mpc. We have examined the multiwavelength properties ofNGC 4450 collectively with those of other low-luminosity active nuclei whichpossess double-peaked broad lines and find a number of common features. Theseobjects are all classified spectroscopically as "type 1" LINERs or closelyrelated objects. The nuclear luminosities are low, both in absolute terms andrelative to the Eddington rates. All of them have compact radio cores, whosestrength relative to the optical nuclear emission places them in the league ofradio-loud active nuclei. The broad-band spectral energy distributions of thesesources are most notable for their deficit of ultraviolet emission compared tothose observed in luminous Seyfert 1 nuclei and quasars. The double-peakedbroad-line radio galaxies Arp 102B and Pictor A have very similar attributes.We discuss how these characteristics can be understood in the context ofadvection-dominated accretion onto massive black holes.Comment: To appear in The Astrophysical Journal. Latex, 15 pages, embedded figures and tabl